1. Field of the Invention
The present invention relates to telephone communication systems, particularly to speakerphone devices and systems and, more particularly to full duplex speakerphone systems.
2. Background Information
Speakerphones (i.e., telephone terminal devices) which permit a local user to communicate via a telephone network with a distally located party without the use of a handset, are common in today's market place. The use of a telephone without a handset is sometimes referred to as "hands-free" operation. Similar to traditional telephones, speakerphones include both a speaker and a microphone. However, in the case of a speakerphone, the speaker and microphone are not secured in a handset that must be held by a user proximate to the user's ear and mouth. Hereinafter, in order to avoid possible confusion, the term "loudspeaker" (as opposed to "speaker") will be utilized when referring to an audio transducer for converting electrical signals into sound energy.
A full duplex speakerphone refers to a telecommunications system capable of simultaneously transmitting audio signals in two directions (that is, it can simultaneously transmit and receive audio signals). In order for a full duplex speakerphone to avoid undesirable audio feedback, a sophisticated process of adaptive echo cancellation of the near end telephone line and acoustic signals is required. Substantially all full duplex speakerphones employ digital signal processors (DSPs) in order to implement adaptive echo cancellation.
Known full duplex DSP based speakerphones utilize a pair of coder-decoders (CODECs) to process the audio signals provided via the telephone line and the microphone. Each CODEC contains an analog-to-digital (A/D) converter and a digital-to-analog (D/A) converter. The first CODEC is coupled to the telephone line and the associated A/D converter digitizes the audio signals provided by the telephone line (i.e., from the distally located party) so that they can be processed before being provided to the speakerphone's loudspeaker. The second CODEC is coupled to the microphone and the associated A/D converter digitizes the audio signals provided by the microphone so that they can be processed before being transmitted via the telephone line. The D/A converter of the first CODEC converts digitally processed signals originating from the microphone to analog signals so that they can be sent to the telephone line. The D/A converter of the second CODEC converts the digitally processed signals originating from the telephone line to analog signals so that they can be provided to the loudspeaker. Each of the first and second CODECs may also include two filters (one coupled to each input and output) to remove high frequency noise so as to avoid aliasing.
A full duplex speakerphone is preferable to a half duplex (or switching) speakerphone. Full duplex speakerphones allow both parties to speak and be heard at the same time (simulating a face-to-face conversation). In contrast, in a half duplex speakerphone similar to the type disclosed in U.S. application Ser. No. 08/454,689 filed May 31, 1996 and assigned to the same assignee as the present application (the entire content of which is incorporated herein by reference), only one channel (i.e., either the transmit or receive channel) is active at any given time. As a result, only one party's voice can be transmitted at a time. Although desirable, full duplex speakerphones are not typically purchased by consumers because the traditional design of these devices make them expensive due to the high cost of the components (e.g. the two CODECs).
Digital, DSP-based full duplex speakerphones are well known. U.S. Pat. No. 4,629,829 to Puhl et al. describes a full duplex speakerphone with two A/D converters, two D/A converters and two adaptive filters. DSP Group, Inc. manufactures chipsets for full duplex speakerphones and each chipset contains two CODECs. However, the cost of the chipsets are relatively high as compared to the entire cost of the components for a full duplex speakerphone. Obviously, it would be a considerable improvement if a full duplex speakerphone could be realized without requiring two CODECs (that is, two A/D converters and two D/A converters).